Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/inetspace_v2_defconfig b/configs/inetspace_v2_defconfig
index 8daf433..e494071 100644
--- a/configs/inetspace_v2_defconfig
+++ b/configs/inetspace_v2_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_NETSPACE_V2=y
 CONFIG_IDENT_STRING=" IS v2"
 CONFIG_SYS_EXTRA_OPTIONS="INETSPACE_V2"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_CONSOLE_MUX=y
 # CONFIG_DISPLAY_BOARDINFO is not set